1. Field of the Invention
This invention relates to a method of making and testing integrated circuits, and a device used to perform such testing.
2. Description of the Prior Art
Integrated circuits (ICs) comprise active and passive elements such as transistors, diodes, resistors, and capacitors, that are interconnected in a predetermined pattern to perform desired functions. The interconnections are effectuated by means of metallization layers and vias. A "via" is a hole through an insulation layer in which conductor material is located to electrically interconnect one conductive layer to another or to an active or passive region in the underlying semiconductor substrate. Present day technology generally employs two metallization layers that are superimposed over the semiconductor wafer structure. Integrated circuits and assemblies have become more complex with time and in a logic circuit, the number of integrated circuit logic units (ICLUs) and interconnects on a given size die have been substantially increased reflecting improved semiconductor processing technology. An ICLU can be a device (such as a transistor), a gate (several transistors) or as many as 25 or more transistors and other devices.
Standard processing to make logic structures (e.g., gate arrays) includes first fabricating as many as half a million transistors comprising a quarter of a million gates per die. Each semiconductor wafer (typically silicon but sometimes of other material such as gallium arsenide) includes many die, for example, several hundred. In one type of gate array, for example, the transistors are arrayed in rows and columns on each die, and each transistor is provided with conductive contact points (typically metal but sometimes formed of other conductive material such as polycrystalline silicon), also arrayed in rows and columns. As is well known in the art, these conductive contact points have a typical center-to-center spacing of about 6 to 15 microns (.mu.m).
In the prior art, the next step is to use fixed masks to fabricate the conductive layers (sometimes called "metallization layers"), to connect together the individual gate-array devices. Typically two or sometimes three metallization layers are used.
After this, the completed die is tested. If any of the devices on the die are defective, that die will fail an exhaustive test and be scrapped. Therefore, the more transistors per die the lower the manufacturing yield. In some cases redundant sections of a circuit are provided that can be substituted for defective sections of a circuit by fuses after metallization. Typically such redundant sections can be 5% to 10% of the total circuit.